microATX (sometimes referred to as µATX, mATX[1] or uATX[2][3]) is a standard for motherboards that was introduced in December 1997.[4] The maximum size of a microATX motherboard is 9.6 × 9.6 in (244 × 244 mm), but some microATX boards can be as small as 6.75 × 6.75 in (171.45 × 171.45 mm).[5] The standard ATX size is 25% longer, at 12 × 9.6 in (305 × 244 mm).

Currently available microATX motherboards support CPUs from VIA, Intel or AMD.

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microATX was explicitly designed to be backward-compatible with ATX. The mounting points of microATX motherboards are a subset of those used on full-size ATX boards, and the I/O panel is identical. Thus, microATX motherboards can be used in full-size ATX cases. Furthermore, most microATX motherboards generally use the same power connectors as ATX motherboards,[6] thus permitting the use of full-size ATX power supplies with microATX boards.

microATX boards often use the same chipsets (northbridges and southbridges) as full-size ATX boards, allowing them to use many of the same components. However, since microATX cases are typically much smaller than ATX cases, they usually have fewer expansion slots.

Most modern ATX motherboards have five or more PCI or PCI-Express expansion slots, while microATX boards typically have only four (four being the maximum permitted by the specification). In order to conserve expansion slots and case space, many manufacturers produce microATX motherboard with a full range of integrated peripherals (especially integrated graphics), which may serve as the basis for small form factor and media center PCs. For example, the ASRock[1] motherboard (pictured right) features onboard Intel GMA graphics, HD Audio audio, and Realtek Ethernet (among others), thus freeing up the expansion slots that would have been used for a graphics card, sound card, and Ethernet card. In recent years, however, it is common even for ATX boards to integrate all these components, as much of this functionality is contained in the typical northbridge/southbridge pair. With the "must-have" functions already present on the motherboard, the need for having many expansion slots has faded, and adoption of microATX has increased even to be used in ATX cases.

A more modern limitation of a microATX case is due to its reduction in drive bays. Current southbridges support up to six SATA devices, in addition to up to four legacy IDE devices. The full range of connectors are commonly found on microATX boards, and can be fully exploited if the board is mounted in an ATX case.

In the build-it-yourself PC market, Micro ATX motherboards in general are favored by cost-conscious buyers, where cost savings for the equivalent feature sets outweighs the added expandability of extra PCI/PCI Express slots provided by the full ATX versions. Since 2006, dual-GPU configurations became possible on Micro ATX motherboards for high-end enthusiast gaming setups, further reducing the need for full ATX motherboards.

In addition, some microATX cases require the use of Low-Profile PCI cards and use power supplies with non-standard dimensions.